Allow different coordinate scheme in DWaveSampler

Something like
```
>>> sampler = DWaveSampler(coordinates=True)
>>> (0, 0, 0, 0) in sampler.nodelist
True
```

There are a few problems with this though:
* This makes the DWaveSampler "topology aware" in a way that it is not currently
* We would need to figure out how to handle `chimera_graph`'s [coordinates](https://github.com/dwavesystems/dwave_networkx/blob/fc537dcee1b25124e411ad8983b23bb36352da3a/dwave_networkx/generators/chimera.py#L66) kwarg and `pegasus_graph`'s [coordinates](https://github.com/dwavesystems/dwave_networkx/blob/fc537dcee1b25124e411ad8983b23bb36352da3a/dwave_networkx/generators/pegasus.py#L62) and [nice_coordinates](https://github.com/dwavesystems/dwave_networkx/blob/fc537dcee1b25124e411ad8983b23bb36352da3a/dwave_networkx/generators/pegasus.py#L80) kwargs. One approach, `coordinates` could switch depending on the topology while `nice_coordaintes` could be consistent from chimera-to-pegasus. This might create issues for future topologies.